1. Field of the Invention
The present invention is related to a microwave attenuator and, more particularly, to a thin-film microwave attenuator capable of attenuating high frequency signals by over 10 dB.
2. Description of the Related Art
Thin-film microwave attenuators have been known in the art at least since the issuance of U.S. Pat. No. 3,227,975 to Hewlett et al. in 1966. As is known in the art, this early design included a thin film of resistive material mounted on insulative material, suspended in a metallic cylinder which was coupled to the outer conductor of a coaxial cable. The resistant material was shaped in a rectangle having a major axis aligned with the axis of the cylinder. Grounding electrodes made contact between opposing sides of the resistive material and the cylinder, while input/output electrodes made contact with the inner conductor of coaxial cables and the sides of the resistive material which were perpendicular to the cylinder.
Numerous modifications have been made to this basic design including having multiple resistive regions mounted on the same insulative substrate; coating the entire surface of the insulative substance, opposite the side on which the resistant material is placed, with a conductive layer to form a ground plane; and shaping the electrodes to simplify connection to a coaxial cable. Examples of some of these modifications can be found in U.S. Pat. No. 3,582,842 to Friedman and U.S. Pat. No. 4,309,677 to Goldman. The attenuator taught by Friedman uses four separate resistive regions, shaped as annular sectors, connected together by a conductive disc and having separate electrodes connected to the outer arcs of each sector. Such a device is not particularly well suited to high frequency applications. The attenuator taught by Goldman uses three resistive regions including, two rectangular ones, each having an input/output electrode connected thereto. Between these two rectangular resistive regions is a conductive region, rectangular in outline, surrounding an annular resistive region. The center of the third, annular resistive region surrounds a hole through the insulative substrate. The hole is coated with conductive material to connect the center of the annular region to a ground plane formed by a conductive surface on the bottom of the insulative substrate. The electrically conductive throughhole is described as minimizing undesired parasitic impedances according to empirical data. The design taught by Goldman also is poorly suited to high frequency operation due to excessive reflections caused by the large number of interfaces between the two rectangular resistive regions.
These and numerous other designs which have been proposed and used for thin-film microwave attenuators are incapable of providing 20 dB attenuation of frequencies at 18 GHz or higher.